Trane RTRA Series Installation Operation & Maintenance page 22

Slide valve movement
Movement of the slide valve piston determines slide
valve position which, in turn, regulates compressor
capacity. Oil flow into and out of the cylinder governs
piston movement and is controlled by the normally-
closed, load and unload solenoid valves.
The solenoid valves receive momentary pulsating «load»
and «unload» voltage signals from the UCM-CLD based
on system cooling requirements. To load the compres-
sor, the UCM-CLD opens the load solenoid valve while
keeping the unload solenoid valve closed. The pressur-
ized oil flow then enters the cylinder and forces the slide
valve to move over the rotors.
The compressor is unloaded when the load solenoid
valve is kept closed and the unload solenoid valve is
opened. Oil «trapped» within the cylinder is drawn out
into the lower-pressure suction area of the compressor.
As the pressurized oil leaves the cylinder, the slide valve
gradually moves away from the rotors.
When both solenoid valves are closed, the present level
of compressor loading is maintained.
Just prior to a normal compressor shutdown, the unload
solenoid valve is energized and the slide valve moves to
the fully-unloaded position, so the unit always starts ful-
ly unloaded.
Oil filter
All RTRA units are equipped with a replaceable-element
oil filter. The filter removes any impurities that could foul
the solenoid valve orifices and compressor internal oil
supply galleries. This also prevents excessive wear of
compressor rotor and bearing surfaces. Refer to the
maintenance portion of this manual for recommended
filter element replacement intervals.
Caution :
The lubricating oils recognised by Trane have been sub-
jected to extensive testing in our laboratories and have
been found to give the required satisfactory results for
use with Trane compressors.
The use of any oil not conforming to Trane required
standards is at the sole responsibility of the user and
could result in warranty cancellation.
Condenser fan staging
The RTRA series offers either the standard or the low
ambient and extra low ambient configuration on the low
ambient option, the fan motors are two speed ones, on
the extra low ambient option, the fan the closest to the
control panel is a speed control fan.
UCM fan staging is function of the difference between
the saturated condenser refrigerant temperature and the
saturated evaporator refrigerant temperature, which in
turn is function of the load and ambient temperature.
Any number of fans can be operating at a given time
depending on these variables.
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